In order to protect the contents of a hopper trailer from dust, rain, road spray, and the like, a tarpaulin is customarily secured around the rim of the hopper, the tarpaulin extending over the contents of the hopper so as to provide a cover. Frequently, a central fore and aft ridge pole is used to give a slope to the tarpaulin to shed rain and the like.
On a gravity discharge hopper trailer, the tarpaulin has heretofore had to be removed prior to unloading to avoid damaging the supporting ridge pole. The downward force exerted by atmospheric pressure on top of the tarpaulin as a crater at sub-atmospheric pressure forms in the underlying commodity during discharge is frequently great enough to rupture the tarpaulin or to fracture the ridge pole and the portions of the trailer to which the ridge pole is fastened.
In some installations, vents have been located along the upper sides and ends of the hopper in an endeavor to alleviate the problem created by the "vacuum" resulting from the crater formation vertically above the hopper door. This expedient suffers from two major flaws. One, since the vacuum forms in the central portion of the hopper and not around the upper margin of the hopper the vents around the periphery frequently have but little effet, particularly where the load is higher than the hopper rim. Two, the vents around the rim of the hopper have allowed rain, dust and road spray to enter the interior of the trailer, causing damage to the commodity.